De-essing?

I've had a vocal with that exact same issue. Initially I tried a couple of de essers but they destroyed the overall sound when I applied the settings required to eliminate the sibilance. I also tried various compressors, sidechained to bring down the volume of the sibilant frequencies but these also couldn't get rid of the nasties without taking away frequencies that the track needed.
In the end I took to surgically editing the waveform by hand. It took time and effort to get it right but it seemed the only way to get the results without destroying the good parts. If you really must keep the track, or can't record another take, then this may be an option for you.

Often when cutting a particular frequency, it can actually give the impression that the octave frequencies above and below have been given a boost (depending on the frquency content of the material).  Cutting these octave frequencies can help tame the sibilance, and I've often found that using a linear phase EQ for such tasks work best.
 
I've no doubt you're already well aware of this, but I thought it could be worth mentioning 

I usually use an eq. (pre-compressor) and play around between 8k-16k. If it's more of a shhh- then a little lower helps between 2-6k. Raise and drop the frequencies one by one and see where the Ms. S's Is's.

I then place a de-esser on that --and then add compression.
I have had to cut and remake many times though as rebel007 said. But with SonarX3 smart tool drawing fades along with the eq/de-esser usually is enough.
Sorry this is so long.
After its all done, I usually give it a nudge at 12k to bring it back to life.

I'm working on a mix and I've got some female vocals that have a super sharp sibilance I'm only just now noticing. Even with stacked de-essers and some creative EQ I just cannot get it tamed. 

 
If it's really essey you aren't going to get rid of it without hearing the tell tale signs of having gotten rid of it.
 
The best way to deal with esses is to accept them as being part of the singer's singing and choose a mic, mic placement, and pre amp that doesn't turn them into ice picking.
 
I've never heard a de-essing procedure that did anything and also sounded good.
 
 

...wicked
I'm working on a mix and I've got some female vocals that have a super sharp sibilance I'm only just now noticing. Even with stacked de-essers and some creative EQ I just cannot get it tamed. The take is otherwise pretty killer, so I'd love to rescue it. Are there any other tricks I can try so it doesn't sound like my singer is singing while wearing a retainer? :-)
 

I usually use Melodyne for De-essing. It's manual labor, but easy to do.
 
Walt

Gain adjustments and possibly fades. If you use EQ as a clip effect, that's more manual adjustment. I would guess you'll get the best results with a mix of techniques with each contributing a slight improvement, which adds up to a bigger improvement.

ooh, I need to try melodyne for de essing.    Always forget about melodyne.
yesterday i used it to adjust timing on a sloppy drum fill and it worked beautifully.  the kicker is, i was editing a bounced mix of the rhythm section.

I tool copy into Wave Lab and set up a narrow EQ, zoom in and apply. Yes this is called manual editing but Wave lab speeds the process up substantially because it's designed to do just these sort of tasks. I'll have to try Melodyne and see if it has the same ease of use. My computers not always up to running it. 

Blarg! Okay I'm striking out here. Tried manual gain envelopes, side-chained the "purposely accentuating the negative" compression technique, and also tried stacking de-essers and eqs. 
 
Just before I call it quits and re-track the vocal I figured I'd toss this out to my nearest and dearest (I'm batting my eyelashes here). A sample of the vocal is here:
https://soundcloud.com/fogpeople/vox-sample
 
If anyone wants to weigh in on whether it's savable or not I'd appreciate it. Maybe a free copy of the whole record when it's done? :-) Especially if you share the secret sauce that fixes it!
 

If we lived close, I'd offer to loan you a nice mic and pre...

Hrm, not hard surfaces, actually custom made baffles around the vox area. Did the whole record like that and I'm four tracks in and vox sound great on everything else. It was a loud track though, so I think we were too close and too hot. 

If it were headphone leakage we'd hear the music in the track, no? 
 
There was a music stand involved, no drums though. I'll double check my export and make sure it was fully dry.
 

I take it melodyne didn't work then ?

Well that's much better, I knew there was something more than what you were describing. 
But first observation, I certainly have trouble hearing what I'd call sibilance now. The doubling made it much worse so I would not add that effect at this point. 
I see there is at least 3 places in the clip where "S" stands out but in a full mix I can't see this being noticed. I hope you not just going with soloing the vocal track and hoping it's perfect. I would go with a full mix first and see if others notice it. Your going to hear it now until you die!! :) 
 
I opened it in Wave Lab and I applied a hi pass at 7,000 with a +7 bump at 1,200 which was to add what will be lost in volume. . I focused in on 
"It's a sideways glance "   then    "it's never touched"   and just rendered those 2 places. 
And it did improve the sibilance a bit. Probably enough to make the grade. Sorry I can't post a sample but just try it yourself. You just need to zoom way in and isolate the "C" or "S"
EQ will certainly diminish the issue but in these cases an automated solution will probably do more damage than good. Manual editing a vocal track is my go to solution for these sort of issues. 
While your in there you can fix all sorts of little things. I use narrow EQ filtering to get rid of sibilance and Plosives. You won't notice a filter on one little 1/2 second syllable like you will on the whole track. 
It's also standard for me to look for quiet passages and use gain to level things out. Just a little here and there does it... you can't overdo these things. A gentle touch. 
Another observation:  
Her enunciation is sort of the problem like she was not exactly sure of the lyrics and is sort of slurring those parts. I found it real hard to understand what was being sung.. but that a good thing in rock and roll..  

Ah, no, I see now. Yes this works well! Okee, I think I'm back on the case here. Thanks so much!
 
I'll chime back in when I finish with the vox and y'all can grade my work. :-)